The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Perpendicular magnetic recording (PMR) is a desired technology for hard disk drive storage due to its associated high storage densities. PMR refers to the vertical alignment of data bits on a recording medium, such as a disk. This vertical alignment provides additional space on the storage medium, thereby enabling higher recording densities. To decode the stored data on the storage medium, a trellis-based Viterbi detector is often coupled to an associated read channel and has certain performance limitations.
Referring to FIG. 1, a traditional read channel detection architecture 10 is shown. The architecture 10 includes a finite impulse response (FIR) filter 12 that receives data samples r(t) and generates an equalized data signal 14. The equalized signal 14 is received by a nonlinear Virterbi (NLV) detector 16 that has an internal Viterbi algorithm to determine the most likely sequence of hidden states. The Viterbi detector 16 produces a preliminary non-return-to-zero (NRZ) data estimates 18 and final NRZ data estimates 20. The final NRZ estimates are produced after a certain amount of latency. This latency is referred to as the Viterbi path memory length. The path memory length is set sufficiently long such that the surviving paths of the Viterbi algorithm merge with high probability and the final NRZ data estimates have good reliability. A surviving path is the most likely path to a particular hidden state.
The preliminary NRZ data estimates 18 have a smaller Viterbi path memory depth than the final NRZ data estimates 20 and thus are referred to as Viterbi early decisions. The early decisions are used to reconstruct the output of the FIR 12 by convolving with a partial response target or reconstruction filter 22. The preliminary NRZ data estimates 18 are received by the reconstruction filter 22 to generate reconstructed FIR outputs 24.
A delay block 26 is connected between the output 28 of the FIR 12 and the reconstruction filter 22. Reconstructed FIR outputs 24 are subtracted from the delayed FIR outputs 30 of the delay block 26 to generate an error signal 32. The error signal 32 is used to calculate error gradients for timing loop, automatic again control (AGC) loop, baseline correction loop, and FIR adaptation loop purposes.
The Viterbi early depth or path length of the preliminary NRZ estimates must be carefully and appropriately selected. When the early depth is too small, the Viterbi early decisions have too many errors, which cause the timing loop and the AGC loop to operate improperly. When the early depth is too long, the timing loop and the AGC loop cannot track fast-varying timing/gain errors.
During PMR of hard disk drives, the magnetoresistive read head produces a zero output voltage at magnetic transitions and a nonzero output in regions of constant magnetic polarity. Thus, the received signal r(t) in PMR has a nonzero DC response. In PMR, a main source of DC distortion or DC noise is the data dependent wandering of the baseline value from AC-coupling in the preamplifier and read channel. The baseline value is a DC reference value. As such, the baseline correction loop is incorporated as a feedback to reduce this DC noise.
The above-described architecture of FIG. 1 has performance limits. The NLV detector is well-equipped to handle data-dependent media noise, but suffers a performance loss when receiving a time variant DC noise, as described above when associated with PMR. This performance loss is primarily due to the relatively long latency of the baseline loop, which fails to adequately compensate for the time variant DC noise.